Fix presumed cut-and-pasteo.

On errors *other* than a short read, we were driving on and processing
the non-data that we didn't read.

Change-Id: I6289ddf31ff7896918a030af9d1261bdc194e7d3
Reviewed-on: https://code.wireshark.org/review/4270
Reviewed-by: Guy Harris <guy@alum.mit.edu>